The selective conversion of methane into its monofunctional derivatives, such as methyl halides or methyl alcohol, is highly desirable, but in practice has not been achieved on any practical basis.
The chlorination of methane is an industrial process practiced on a large scale. The reaction is a strongly exothermic one which takes place via free radicals and is generally conducted without supplying heat and usually in the absence of a catalyst at 400.degree.-450.degree. C. under slightly elevated pressures. The chlorination is normally thermally initiated via homolysis of chlorine molecules to chlorine atoms; the process can also be operated photochemically. For surveys of these processes, it is appropriate to refer to F. Asinger "Paraffins. Chemistry and Technology", Pergamon Press, New York, 1968; M. L. Poutsma "Methods in Free Radical Chemistry", Vol. II, E. S. Huyser, Ed., M. Dekker, New York, 1969; and R. Weissermel and H. J. Arpe "Industrial Organic Chemistry", Verlag Chemie, 1978, pp. 46-47. By these reactions, all of the possible chlorinated methanes are usually formed together when an equimolar Cl.sub.2 /CH.sub.4 ratio is employed: ##STR1## If methyl chloride is the preferred product, a large excess of methane (approx. tenfold) must be used, as methyl chloride is more rapidly chlorinated than methane under free radical conditions. There are normally many by-products of the chlorination of methane, such as hexachloroethane and small amounts of trichloroethylene.
Methyl alcohol is increasingly important not only as a chemical raw material and building block for such products as formaldehyde, acetic acid, vinyl acetate, ethylene glycol and others, but also via its condensation reactions to give gasoline or hydrocarbons, such as olefins, aromatics and the like. Its direct use as a transportation fuel is also gaining importance. A whole scope of so-called C.sub.1 chemistry is based primarily on methyl alcohol.
Methyl alcohol, once produced from wood fermentation (i.e., wood alcohol), is, at the present time, nearly exclusively produced from CO and H.sub.2 (synthesis gas) derived from coal or natural gas. Coal or methane first must be converted in an energy consuming step into syn-gas, which is then, in a second energy consuming step under pressure and generally forcing conditions, converted into methyl alcohol. Clearly, direct oxidative conversion of methane into methyl alcohol would be highly desirable. Despite continued efforts no such process was, however, previously achieved on a practical scale.
The oxidation of methane generally is not selective. In the past, many attempted oxidations concentrated on manufacturing formaldehyde from methane. The low rate of reaction of CH.sub.4 at temperatures below 600.degree. C. coupled with the high rate of decomposition of formaldehyde above 600.degree. C. is probably the reason that no industrial process has been developed to date. Decomposition of formaldehyde could only be avoided be extremely short residence times. Such a process has been recently described involving partial oxidation of methane to methyl alcohol and formaldehyde. The residence time is 1.55.times.10.sup.-3 sec. and the pressure 60 atm., respectively (Huels). However, oxidation of methane, similarly to chlorination, is free radical chain reaction, which explains the observed lack of selectivity.
I have previously described in the Journal of the American Chemical Society, Vol. 95, 7686 (1973) that, under specific conditions, alkanes can undergo electrophilic chlorination and chlorinolysis. With a SbF.sub.5 catalyst in SO.sub.2 ClF solution at -78.degree. or at room temperature with a reaction time of 24 hours, methane was transformed qualitatively to methyl chloride. No practical yields were obtained. AlCl.sub.3 catalyst gave under similar conditions 1% methyl chloride. These reactions clearly did not represent a practical method for the chlorination of methane.